How to alter webform programmatically using hook_ENTITY_TYPE_prepare_form()

Using Webform 6.2.0-beta5 on D10.0.9.

I have created a custom webform with a module; this is working perfectly.

Now I need to make several changes to the webform; some values will be calculated within my custom module, used as the default value for fields, and then the fields will be hidden.

With this code I can see the $webform details:

function mymodule_webform_submission_prepare_form(WebformSubmissionInterface $webform_submission, $operation, FormStateInterface $form_state) {
  $webform = $webform_submission->getWebform();
  if ($webform->id() == 'custom_webform_id') {
    \Drupal::messenger()->addStatus($webform);
  }
}

How do I now, for example:

1. Add a value to a webform field
2. Hide that webform field from the user

This will have the effect of programmatically adding values to the webform fields.

Thanks

Goal: Add some hidden fields to a webform, calculate the values for those fields in custom code, and then submit them with the webform.

You could use a hook for this, but I think there's a better approach: standard Webform hidden fields with a custom token.

Webfoorm supports creating hidden fields in the UI, so just create the fields you need there.

Then, you can set the value of the hidden fields to a token-- this is where your custom code comes in. You can create tokens in a custom module, and then, after you rebuild the cache, your tokens will be available for use in webform.

Here's a tutorial about creating tokens, and the Examples module also has sample code for creating tokens.